The main objective of the proposed research is to study the role of thyroid hormones in surfactant synthesis and/or secretion in the rat. Towards this end, studies will be conducted to examine the maturation of the surfactant system utilizing an enzyme marker for surfactant (identifiable biochemically, electrophoretically and cytochemically) and to determine the relationship of these developmental changes to the appearance of triiodothyronine (T3) and thyroxine (T4) in the lung. Specifically, information will be obtained on (1) the ability of T3 and T4 to accelerate lung development; (2) the dependence of lung development on normal thyroid and/or adrenal function; (3) the time in development when the lung is most responsive to T3 and T4; (4) the effect of T3 and T4 on type II cells studied in tissue culture; and (5) the factors responsible for the postnatal appearance of pulmonary thyroid hormones. If the results of this study indicate that defects in fetal and/or neonatal thyroid function can produce deficits in lung development and surfactant synthesis and/or secretion, then another mode of prevention and/or therapy of RDS, a disease characterized by lung immaturity and deficient surfactant, may be possible.